Sputtering target assemblies are used in the formation of a wide variety of products. Some typical substrates on which the target material of sputtering target assemblies is deposited include items such as semiconductor devices, compact discs (CDs), hard disks for use in magnetic disk drives, solar cells, optical devices such as flat panel displays, and non-aqueous lithium secondary batteries.
A typical sputtering apparatus, such as one used in the production of the above described items, comprises a vacuum chamber inside of which are positioned the sputtering target assembly and the substrate. The target of the target assembly is electrically configured to be an electrode with a large ion flux. An inert gas, often in conjunction with a reactive gas, is introduced into the chamber and ionizes when power is supplied to the target/electrode. The positively charged inert gas ions collide with the target causing atomic sized particles to be ejected from the target. The particles are then deposited on the surface of the substrate as a thin film.
Because of this electrical configuration, the target can become very hot and needs to be cooled. In a typical sputtering apparatus, the cooling is provided by a water-cooled backing member to which the target is attached by an attachment layer. In some sputtering systems, a rectangular target and backing plate are used, while in other systems, the target and backing support are cylindrical in shape.
A trend in the manufacturing of flat panel displays and other devices such as those described above is to manufacture many devices on a very large substrate, much like smaller semiconductor devices are manufactured on wafers. For example, flat panel display manufacturers would like to be able to process square or rectangular flat panel display substrates having surface areas on the order of approximately 1200 square inches (7742 square centimeters) to 6000 square inches (38,700 square centimeters) or more. Some of these large substrates are currently being processed using large rectangular sputtering targets that are indium bonded to a backing plate. However, cylindrical sputtering targets can provide advantages in terms of increased film deposition rates, a lower propensity for nodule formation, and higher target material utilization than the corresponding sputtering of planar targets. The production of rotary (e.g., cylindrical target) sputtering target assemblies that are long enough for use with substrates having large surface areas (e.g., on the order of approximately 1200 square inches or more), however, present special bonding considerations and problems (e.g., there are difficulties in both the initial manufacture of a cylindrical sputtering target assembly of, for example, 0.5 to 4 meters or more, and in the manufacture of sputtering target assemblies that are of sufficiently high enough quality to meet the increasingly more stringent standards associated with film formation).
Manufacturing methods for sputtering target assemblies include a method of molding, from raw material powder, sputtering targets (i.e., target bodies used in the sputtering target assembly) and sintering the molded body. Moreover, quality demands for sputtering targets include, for example, purity control; suitable structure (e.g., crystalline structure); appropriate grain size distribution; uniform composition distribution, and a high density. Here, the relative density means a ratio between a density of a porous material and a density of a material having the same composition in a state which has no air holes.
When the sputtering target is configured of a sintered body of a raw material powder, there are numerous factors (physical and chemical attributes) both in the production and in the use of such targets that can lead to problems. For example, such ceramic material targets can easily be broken during the formation of the sintered body, and it is difficult to manufacture a sintered body having a high density and good homogeneity constantly. The sputtering target is also placed in a harsh environment and subject to potentially degrading influences during use (again physical and chemical), and thus has to be formed in an effort to avoid such degradation during use (e.g., undesirable nodule formation, chipping, peeling, cracking, separation from its support, etc.). Moreover, the sputtering target assembly, itself, needs to be of high quality to avoid issues with debonding, uneven surfaces relative to multiple target bodies supported on a common backing, nodule formation, etc., due to lower quality attributes generated in the target assembly manufacturing process.
Because of the increased difficulty in the production of hollow cylindrical sputtering targets as they get larger, to satisfy long length requirements in sputtering target assemblies, large sputtering targets generally consist of a common backing tube on which a plurality of target bodies (or segments) are bonded, i.e., the target bodies are attached to the backing support in series (typically with some intermediate spacing) by a bonding material which can be, for example, a metal, or metal alloy, having a low melting temperature, also called a metallic solder, or any other kind of electrically and thermally conductive adhesive, such as a filled elastomer.
The function of the support is to provide for electrical power transfer, mechanical strength, and heat transfer to the cooling water, and it allows the target to be mounted in the sputtering source. As the cylindrical target bodies or segments, in the form of cylinders, are often bonded on the support by soldering, there is in most cases a mismatch in coefficients of thermal expansion between the target material and the support materials, which results in thermal stresses on the solder layer and on the interfaces between target segment and solder as well as between solder and backing body, especially during cooling down after bonding or soldering of the target, but also during usage of the target in the sputtering process. In addition there can be a volume contraction of the solder when it solidifies, which also leads to interface contraction stress.
For example, during sputtering operation the cumulative thermal and contraction stress often leads to strong and uncontrolled delamination of the solder layer from either the backing body or the target segment material or from both. Where delamination over larger surface areas occurs, heat dissipation from the target segment to the support is minimal, which leads to local overheating, provoking more uneven thermal stress and eventually cracking of the target segments during sputtering. The bonding of hollow cylindrical target segments in series along the backing tube can also lead to breakage and added rework expense. For example, there can be breakage during the initial placement of cylindrical target bodies on the backing support, particularly when there is, for instance, lacking proper wetting techniques. Poor or imprecise assembly can also lead to low quality regions on the target assembly (e.g., non-aligned outer surfaces or outer diameter surfaces, unseen increased cracking, leakage regions and/or spacing issues) that can lead to the aforementioned operational issues of cracking, nodule formation, etc.
Various techniques have been proposed for bonding targets to their support that include bonding certain types of circumferentially arranged hollow ceramic target bodies (e.g., ceramic targets such as ITO and AZO targets) relative to an interior support tube with such techniques being inclusive of the use of a metal solder binder (e.g., indium), which is provided in conjunction with radiative heating that is used to raise the temperature of the ceramic cylinder and backing tube at the time of assembly, and maintain the temperature of the combination above the melting point of the solder, as molten solder is poured into the gap formed between the target and support. This radiative heating can be accomplished with vertical clam shell heater designs.
Under such radiative heating, temperature gradients at the top and bottom can only be controlled with multi-zoned heaters that are large in size. This size limits access to the target assembly being bonded, both physically and visually, and can limit the ability of an assembler to detect, for instance, leaks of solder between segment gaps or cracked ceramic walls early enough to minimize wasted time and material due to extensive rework. The control of temperature gradients, even with multi-zone heaters, is inadequate and can lead to local hot spots and thermal shock which can contribute to the cracking of the target body and solder oxidation. Proper heating during binding of a sputtering target to its backing support is thus an example of an area relative to the problems described above that arises relative to the formation of target assemblies, that, if not addressed, can raise potential issues during the manufacture of products using sputtering target assemblies (e.g., poor quality thin film products that are formed in conjunction with an inadequately operating sputtering target assembly).